Air Heat flow

Operating principle of the air energy meter

The air energy meter measurement system, invented by Jens Amberg and protected by multiple patents, solves the task of measuring flow rate and heat flow in air or flue gas. In doing so, it meets all requirements of the product standard DIN 94701 (“air energy meter”). Its key advantage lies in the fact that the air energy meter is a compact measurement system that has been proven in practice in more than 700 projects and has been developed with all necessary interfaces and software functions.

One of its main functions is flow rate measurement. Any differential pressure device (such as pitot tubes, orifice plates, Venturi tubes, etc.) can be connected to the air energy meter – preferably, of course, the in-house probes such as Luftmeister Freiburg or Luftmeister Kassel. How Luftmeister deals with typically asymmetrical flow profiles and other application-related challenges is described in the Air flow rate section.
The evaluation of the differential pressure is performed using a cascade of two differential pressure sensors, ensuring accurate measurement of both high and low flow rates. In most applications, it is also essential to output a density-compensated value (standardised volume flow, dry or moist mass flow, etc.). Air density is determined based on its three components: temperature (°C), humidity (%RH) and absolute pressure (hPa). The absolute pressure sensor is integrated into the air energy meter, while temperature and humidity are connected as external sensors – providing maximum flexibility for different applications.

In addition, the air energy meter determines enthalpy (kWh/kg), i.e. the amount of thermal energy carried by 1 kg of air. This value is also calculated using temperature, humidity and absolute pressure. Based on mass flow rate and enthalpy, the thermal power (kW) is then calculated – a key control and monitoring variable for thermal efficiency.

Furthermore, the air energy meter features three meter registers. In addition to air volume (m³), the air-side heat energy meter (kWh, “red”) is incremented whenever the thermal power is greater than zero. During periods when the thermal power is negative, the separate and independent air-side cooling energy meter (kWh, “blue”) is incremented instead.

The measurement system can be conveniently parameterised using the powerful Luftmeister Tool software. The nine logger channels of the air energy meter can also be read out via this software, which additionally supports sensor adjustment and logbook data retrieval.

To accommodate the wide range of applications, the air energy meter provides numerous interfaces. It can output up to 11 analogue signals, up to 10 switching or pulse signals, as well as the digital bus signals M-Bus or Modbus RTU. The system is complemented by a high-performance touch display with an extensive operating menu.

Finally, a special feature of the air energy meter should be mentioned: it supports the interconnection of multiple devices via the digital EZ bus. This allows several measurement points to share the same enthalpy measurement location (for example, requiring outdoor air enthalpy to be measured only once). In addition, mass flow rates can be added or subtracted to represent branching systems.

Precise measurement of density, enthalpy and humidity

Air density and enthalpy are key parameters for accurate flow rate and heat flow measurement. Both quantities are determined by temperature, humidity and absolute pressure. While temperature plays a central role for both density and enthalpy, absolute pressure primarily influences density (but hardly enthalpy), whereas humidity has a decisive influence on enthalpy (but not on density).

Among these parameters, humidity measurement represents the greatest metrological challenge – while at the same time being a crucial parameter for enthalpy and thus for heat flow measurement. In addition, humidity is a key quality criterion of air (for example ultra-dry air in battery manufacturing, comfort zone ventilation in HVAC applications, or moisture control in drying processes). For this reason, Luftmeister uses a total of four different humidity measurement systems, each providing high precision and robustness within its typical application range.

The air energy meter measurement system is capable of detecting residual moisture levels down to –120 °C dew point as well as high humidity levels at, for example, 30 °C, 170 °C or 380 °C. To ensure reliable operation in industrial processes with contaminated air and sometimes very high flow velocities, Luftmeister protects the humidity sensors using special protective shields and purge systems.

To cover the full range of applications, the air energy meter supports both input connection and signal output in various humidity parameters – from relative humidity (%RH) and dew point (°Ctd) to absolute humidity (g H₂O/m³ air), humidity ratio (g H₂O/kg air) and dew point depression (K).

Measurement of state changes

Conditioned air and process air undergo gradual changes in their conditions, which are typically defined by temperature and humidity. For example, outdoor air is usually routed through a heat recovery system (HRU) before passing through heating coils, cooling coils and, if applicable, humidification or dehumidification stages. In process applications, air conditions also change continuously, and exhaust air rarely exhibits the same state as supply air.

All of these state changes – or individual stages thereof – can be captured by the air energy meter measurement system. The key feature is that this is a continuous measurement, making it possible, for example, to determine the actual thermal contribution delivered by a heat recovery unit at any point in time throughout the year. It also becomes clear which room loads actually occur in different seasons – an ideal criterion when deciding to replace an existing system with a better-matched new installation.

This measurement solution offers particularly high efficiency potential in applications involving costly process air. If the relevant parameters for optimal heat recovery performance can be identified and continuously controlled on the basis of these measurements, substantial savings can be achieved on a regular basis.

In addition, the measurement of exhaust air heat often becomes a key focus when existing heat recovery systems are missing or undersized. Based on Luftmeister’s heat flow measurement data, new heat recovery systems can then be dimensioned appropriately.

Heat flow measurement in process air

In addition to the state changes described above, there is often strong interest in measuring the heat flowing through selected air ducts in process air or flue gas applications. How much heat flows from a furnace system to a dryer? How much heat is discharged via a stack or an exhaust duct? And more generally: which heat flows are absorbed by a process engineering system, how are they redistributed internally and to what extent, and which heat flows are emitted?

From a physical perspective, this approach does not focus on changes in enthalpy that are relevant for state analyses (such as the enthalpy upstream and downstream of a heat exchanger). Instead, the measured process enthalpy is referenced to an enthalpy zero point, for example the enthalpy at 0 °C, which is conveniently defined as 0 kWh/kg.

The air energy meter also supports these measurement tasks professionally, enabling process engineering systems to be monitored, controlled and optimised on the basis of reliable heat flow data.

Efficiency optimisation through quality measurement series

In the field of process air and process engineering, systems often need to be operated efficiently not only at full load. For example, ceramic kilns are not used exclusively for the main product at 100% utilisation, but are also operated for secondary products and under partial-load conditions.

In this context, the air energy meter has proven particularly effective, as these operating scenarios – which typically become more frequent over time – offer significant efficiency potential. The approach is as follows: the relevant air ducts are equipped with air energy meter measurement points. Subsequently, so-called quality measurement series are carried out. This means that, for all relevant product and partial-load combinations (e.g. “Product B at 100 / 80 / 60% utilisation”), variations of the heat flow are tested – always ensuring that product quality remains uncompromised.

In this way, the most energy-efficient operating modes are identified step by step. The resulting parameters can then be stored in the higher-level process control system for automated control.

Dryer optimisation using the air energy meter

Most drying systems are based on the ability of air to absorb moisture. Regardless of whether the air flows over the surface of the material being dried (hot-air dryers), passes through bulk material (belt dryers) or surrounds rotating bulk material (drum dryers), the objective is always for the exhaust air to remove excess moisture. Even more complex process dryers, such as spray dryers (where liquid is atomised and dried with hot air) or fluidised-bed and jet dryers, ultimately rely on the moisture absorption capacity of air.

The air energy meter supports the efficiency control of dryers in two ways. First, it displays the time profile of thermal power (exhaust air mass flow multiplied by the enthalpy difference between exhaust air and supply air). This makes it possible to identify optimal time profiles (see quality measurement series above) and store them as control setpoints.

Second, the air energy meter can measure the air mass flow and, at the same time, the humidity ratio of the air (g H₂O per kg of air). By multiplying these two values, the actual moisture removal rate (litres per hour) is obtained directly. This makes it clear how much thermal power is required per litre of water removed, allowing the most energy-efficient operating mode to be determined and stored as a control setpoint.